Srithai, B.; (2007) Geochemistry and petrogenesis of corundum from the Bo Ploi deposit, Kanchanaburi, Thailand. Doctoral thesis , University of London.

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Abstract

Corundum from the Bo Ploi deposit, Kanchanaburi, Thailand and its associated Tertiary alkali basalt were studied in terms of mineralogy, geochemistry and petrogenesis to ascertain the genetic link between these materials. Gem quality corundum, sapphire variety, is composed of almost pure AI2O3 with a small but significant amount of FeO and TiC>2. Optical spectroscopic studies reveal that the blue colour is caused by Fe2+ - Ti4+ intervalence charge transfer. The syn-genetic inclusions are mineral inclusions, including, feldspar, zircon, spinel, pyrochlore, boehmite, rutile, monazite, and opaque minerals fluid inclusions and silicate melt inclusions. Microthermometric studies of fluid and silicate-melt inclusions trapped in these sapphires suggest that they grew from silicate-melt under H2O-, COrrich and hypersaline brine conditions at the temperature approximately 800- 1000 C. The silicate-melt inclusions are trachyandesitic compositions indicating that the Bo Ploi corundum is magmatic origin forming from the extreme fractionation of trachyandesitic magma with additional alumina contamination from crustal materials and brought to the surface by later eruption of the Bo Ploi basalt. The sapphires oxygen isotope chemistry, 8180 6.5-9.6%o, suggests a mingling of crustal and mantle oxygen isotope thus confirming the role of crustal contamination. The Bo Ploi Tertiary basalt is a within-plate magma of basanite composition, which differentiated through the fractional crystallisation process without a significant crustal contamination, 5 O 6.0-6.9%o, although numbers of crustal xenoliths including granites, nepheline syenite and quartzite are found in it. The basanites also accommodated mantle xenoliths, mainly spinel lherzolite, which provided P-T scheme based on thermodynamic calculations of coexisting mineral equilibria in the range of 11 to 24 kbar and temperature of 1000 to 1200 C correspond to the depths of 36 to 79 km in the upper mantle.

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